1. Field
The present disclosure relates to a quantum cryptography field. More particularly, the present disclosure relates to an apparatus and method for stabilizing a quantum cryptography system, for maintaining optimal system performance.
[Description about National Research and Development Support]                This study was supported by the Ministry of Science, ICT and Future Planning, as Project No. 1711022147.        
2. Description of the Related Art
A quantum cryptography system allows two separated stations, namely a sender (Alice) and a receiver (Bob), to distribute one-time pad keys safely. In the quantum cryptography system, quantum key distribution (QKD) uses a weak light signal (for example, single photon) sent through a quantum channel, which has a relation with sharing of a key between the sender (Alice) and the receiver (Bob). At this time, the stability of key distribution is based on a quantum mechanics principle, defining that, if any quantum system in an unstable state is measured, the unstable state will be corrected. As a result, if a pirate listener (Eve) attempts to disturb or measure a quantum signal, an error occurs at the sending signal, thereby revealing the presence of a private listener.
The quantum cryptography system should align various variables according to time and maintain the arrangement in order to optimize system performance. For example, in order to detect a single photon at an expected photon arrival time, the quantum cryptography system should transmit a gate pulse for gating a photon detector at a time suitable for the photon detector to synchronize the photon arrival timing and the photon detection timing. In other case, an operation time of laser pulse may also be adjusted to synchronize the photon arrival timing and the photon detection timing.
However, this timing is drifted due to various environmental factors (for example, a change of a light path caused by shrinkage of an optical fiber due to the temperature change of an actual environment), which deteriorates a photon count. As a result, a transmission rate and a key distribution rate of the quantum cryptography system are reduced, and a bit error rate (BER) increases, thereby deteriorating system performance. Therefore, if the system performance deteriorates, the quantum cryptography system should automatically arrange the arrival time or the like to a photon detector of the gate pulse to stabilize the system, so that the system may operate with optimal performance again.